(a) Field of the Invention
The present invention relates to an electric double layer capacitor (EDLC) and, more particularly, to the structure of the thrust plates and the mounting gasket of the EDLC having higher characteristics and a larger capacitance with smaller dimensions.
(b) Description of the Related Art
The EDLCs have been used in a variety of applications, and now are expected for new applications such as a power source for driving a cell motor of a car in combination with a lead battery, and an auxiliary power source in combination with a solar battery. The EDLC is now being developed to have a larger storage capacity with smaller dimensions by taking advantage of the structure of the EDLC having a small thickness.
FIGS. 1A and 1B show conventional EDLCs in cross section. The EDLC includes a plurality of unit capacitors or unit cells stacked one on another and each including a pair of polarizing electrodes 30, a porous separator 40, a pair of current collectors 20 made of conductive sheet, and a gasket 50. Each unit cell is supported by a cylindrical mounting gasket 50 for encircling the polarizing electrodes 30, which sandwich therebetween the porous separator 40 and is sandwiched between the pair of current collectors 20. The current collectors 20 disposed outermost side of the stacked unit cells are in electric contact with a pair of metallic terminals (not shown) each having an external lead terminal.
The polarizing electrodes 30 are impregnated with an electrolytic solution which may be an aqueous or organic solution, and sealed by an packaging overcoat not shown in the drawings.
The withstand voltage of the EDLC is generally limited by the electrolysis voltage of the electrolytic solution, and thus a specified number of unit cells are stacked for serial connection depending on the desired withstand voltage, with one of the current collectors of the unit cell being shared with the adjacent unit cell, or in contact with one of the current collectors of the adjacent unit cell.
Patent Publications JP-A-55-107225, -5-299295, -8-83596 and -11-135382 describe EDLCs having a lower internal resistance and excellent resistance against mechanical impacts. In these EDLCs, each unit cell having a pair of carbon electrodes impregnated with electrolytic solution and sandwiching therebetween a porous separator is encapsulated by a laminated packaging overcoat made of organic polymers having a metallic film as a core member under a vacuum ambient.
Patent Publication JP-A-5-299295 describes a EDLC wherein a unit cell is formed by thrusting the capacitor elements received in an envelope between a pair of thrust plates, followed by screwing the same.
For the EDLCs, it is proposed or suggested in a variety of publications such as mentioned above to reduce the contact resistance in the unit cell as well as stacked unit cells, in view of the trend for obtaining a higher storage capacity with smaller dimension.
In the proposed EDLC, wherein the contact resistance is reduced by using the pair of thrust plates, for example, there is a problem in that it is difficult to achieve a uniform close contact in fact for reducing the contact resistance. In addition, the stacked unit cells are likely to be misaligned at the interface therebetween during the thrusting operation under the vacuum ambient.
Thus, in the conventional EDLC, there occur damages such as split, distortion, or ununiform contact of the electrode layers, and damage or crack of the package. In short, desired equivalent serial resistance (ESR) characteristics or stability thereof as well as desired reliability of the product are not achieved in the conventional EDLC.
In view of the above problems in the conventional EDLCs, it is an object of the present invention to provide a structure for the EDLC which is capable of allowing smaller dimensions and having a larger storage capacity, and reducing and stabilizing the ESR etc. characteristics of the EDLC.
The present invention provides, in one aspect thereof, an electric double-layer capacitor (EDLC) including: a cell assembly including at least one unit cell having a pair of polarizing electrodes impregnated with electrolytic solution, and a porous separator sandwiched between the polarizing electrodes, a pair of current collectors sandwiching therebetween the unit cell, and a pair of lead terminals in electric contact with the current collectors; a pair of thrust plates each disposed on a corresponding one of the current collector for thrusting therebetween the cell assembly, a mounting gasket for receiving the cell assembly together with the thrust plates; and a packaging overcoat for encapsulating the cell assembly, thrust plates and the mounting gasket, each of the thrust plates having a slanted side surface in contact with the packaging overcoat.
The present invention provide, in another aspect thereof, an electric double-layer capacitor (EDLC) comprising: a cell assembly including at least one unit cell having a pair of polarizing electrodes impregnated with electrolytic solution, and a porous separator sandwiched between the polarizing electrodes, a pair of current collectors sandwiching therebetween the unit cell, and a pair of lead terminals in electric contact with the current collectors; a pair of thrust plates each disposed on a corresponding one of the current collector for thrusting therebetween the cell assembly, a mounting gasket for receiving the cell assembly together with the thrust plates; and a packaging overcoat for encapsulating the cell assembly, thrust plates and the mounting gasket, each of the thrust plates having an effective thrusting area which is equivalent to or smaller than an effective area of the current collector.
In accordance with the present invention, the configuration of the thrust plates affords excellent device characteristics to the EDLC such as an excellent ESR characteristic.